In marine operations involving subsea work, it's often necessary to deploy a structure, vehicle or piece of equipment in different aquatic locations, sometimes far from land and terrestrial support systems. Traditional deployment methods use various types of assist vessels having heavy cranes, heave compensation systems, reinforced cages/protective systems, “A” frames, moon pools and lowering decks. The structure being deployed is typically mechanically coupled (directly or indirectly) to the vessel deploying it, as the load is lowered into and recovered from the water. The traditional deployment and recovery systems attempt to resolve the fundamental problem of matching, or minimizing, the relative motion of two spatially separate but mechanically linked masses in a dynamic environment such as at the sea surface. Entry of the load into and exit of the load from a turbulent “splash zone” at and near the water surface can be treacherous and may damage the load or the launch/recovery vessel, and may as well produce unsafe conditions for personnel and equipment in the area.
In one exemplary launch and recovery system disclosed in PCT International Patent Publication WO 2013/072690, a subsea payload is lifted from and deployed to an undersea location using a lift line supported by a heave-compensating winch on a surface vessel. Movement of a submersible launch unit is controlled by means of on-board thrusters. Vessel and crew time are expensive, and the expense incurred as a result of launch and recovery vessel and crew requirements may limit the frequency of launch and recovery operations, particularly in remote locations.
One system that avoids mechanically joining heavy dissimilar objects (such as a launch vessel and a submersible vessel or payload) at the splash zone places the vehicle or payload on a “sinking barge” and tows the barge/payload to a dive location, where both the barge and the payload are sunk, as a unit, under controlled conditions, generally using an active depth control system. The payload/vehicle is deployed by releasing it from the barge transport component at a desired depth where the sea conditions are steady and manageable. The few existing systems that use this type of subsurface launch and recovery approach require the use of an active manually operated depth control system, and they generally require the use of divers to detach the vehicle from the barge during deployment and to re-attach the vehicle during recovery. Personnel are also generally required to operate the active depth control system during launch and recovery.
A submersible launch, recovery and transport vehicle (LRT) of the aforementioned type was developed for transporting and deploying research submarines in rough waters in connection with the Hawaii Undersea Research Laboratory (HURL). The LRT is a twin-hulled, submersible platform upon which a submarine may be positioned and secured. It is towed on the surface by a support vessel to a desired dive site, and the LRT and submarine are both submerged, under the control of a diver pilot, to maintain a stable hover at a depth of 50-60 feet. The submarine is released from the platform by diver(s), and the LRT maintains at hover awaiting return of the submarine, or returns to the surface. Tanks having high-pressure air (e.g., 3000 psi) are used in the active buoyancy system, which submerges and raises the platform, and also maintains the platform at a desired depth. The use of high-pressure air and tanks requires the use of high pressure fittings and results in long tank fill and evacuation times, increasing the cost and complexity of the system. While this system allows subsea launch and recovery of submersible vehicles from a subsea location that is isolated from surface conditions (waves, etc.), it requires significant assistance in terms of personnel, and any failure of the active depth control system may result in damage to, or loss of the LRT or its payload.
Systems for submerging work platforms and for supporting submerged work platforms during underwater activities are also known. In one system disclosed in U.S. Pat. No. 5,507,596, a support system for supporting a submerged work platform using one or more vessels uses a plurality of cables connected between the surface support vessel(s) and the underwater platform. Several individually controlled cables are used to provide a desired number of degrees of freedom of control vis a vis the work platform. The surface vessel(s) (e.g., ship(s) or barge(s)) are subject to surface sea motions, and the motions of the support structure(s) are sensed and the length of the cables is actively adjusted to maintain the work platform stationary, even as the support vessel(s) move at the sea surface.
Notwithstanding the existence of various launch and recovery systems, and of various schemes for supporting underwater platforms, there remains a need for a simple vessel and payload launch and recovery system that permits the safe deployment and recovery of equipment and vessels in heavy sea conditions, and that does not require substantial vessel or personnel support or active depth control systems.